Dual-amplifier circuit for optical signals

ABSTRACT

A system for amplification of optical signals for optical measurement instrumentation is disclosed. The system may include a first logarithmic amplifier circuit in a first amplification path, and a linear amplifier circuit and a second logarithmic amplifier circuit coupled in series in a second amplification path. The first and second amplification paths may receive an input signal from a photodiode and provide amplified signals, in parallel, to a selection circuit, which may select one of the outputs of the first and second amplification paths based on one or two power thresholds. The selection circuit may then provide the selected output to a measurement circuit or device. In some examples, the system may also include a sampling circuit to sample the outputs and an analog-digital conversion circuit to digitize the outputs before selection. The power threshold(s) may be determined based on a saturation level of the linear amplifier circuit.

TECHNICAL FIELD

This patent application is directed to optical measurementinstrumentation, and more specifically, a dual-amplifier circuit foroptical signals with enhanced speed and dynamic range.

BACKGROUND

Optical measurement instrumentation, such as optical spectrometers oroptical spectrum analyzers (OSAs), optical power meters, fiberopticmonitoring devices, and similar ones play an important role intelecommunication and other optical technologies. Optical measurementinstrumentation may include photodetection amplification circuits.Photodetection amplification circuits may include linear gain-switchedand logarithmic-gain amplifier circuits, both of which may havechallenges in detecting low power, high frequency optical signals.

BRIEF DESCRIPTION OF DRAWINGS

Features of the present disclosure are illustrated by way of example andnot limited in the following Figure(s), in which like numerals indicatelike elements, in which:

FIG. 1 illustrates a block diagram 100 of a detection portion of anoptical measurement device, according to an example.

FIG. 2 illustrates a schematic diagram 200 of an amplification portionof an optical measurement device, according to an example.

FIG. 3 illustrates a block diagram of an optical system 300 including anoptical source or signal to be measured and a measurement system thatmay employ a linear amplifier and a logarithmic amplifier detector,according to an example.

FIGS. 4A-4B illustrate graphs 400A and 400B representing input andoutput signals of a linear amplifier, a logarithmic amplifier, and acombination of thereof, according to an example.

FIG. 5 illustrates a diagram illustrating selection of differentamplifier configurations based on a power threshold, according to anexample.

FIG. 6 illustrates a flow chart of a method for employing a combinationof linear and logarithmic amplifiers to amplify optical signals,according to an example.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the present disclosure isdescribed by referring mainly to examples and embodiments thereof. Inthe following description, numerous specific details are set forth inorder to provide a thorough understanding of the present disclosure. Itwill be readily apparent, however, that the present disclosure may bepracticed without limitation to these specific details. In otherinstances, some methods and structures readily understood by one ofordinary skill in the art have not been described in detail so as not tounnecessarily obscure the present disclosure. As used herein, the terms“a” and “an” are intended to denote at least one of a particularelement, the term “includes” means includes but not limited to, the term“including” means including but not limited to, and the term “based on”means based at least in part on.

As mentioned herein, some photodetection devices may include lineargain-switched amplification following a photodetector, for example, aphotodiode. When detected optical power levels are low, a lineargain-switched amplifier may respond by switching to a highest gain rangeof the linear amplifier circuit. As the output signal rises, so does theoutput amplified photo-current, until the circuit reaches the maximumrange the gain-stage. When this threshold is reached, the linearamplifier circuit may dynamically switch to a lower gain range. Inaddition to the amplification limit of the linear amplifier circuit,fast optical events traversing the switching threshold may be hidden bythe circuit switching. Furthermore, high slew-rate changes spanningmultiple gain stages may present additional challenges in detecting fastsignal transitions, for example in finely resolved spectral features inan optical spectrum analyzer (OSA).

Other photodetection devices may include logarithmic amplifier-basedamplification, which may have a single gain-stage covering an entireinput signal span from very low light levels to very high light levels.In addition to logarithmic amplifier circuits being able to obviate aneed for gain switching and be capable of high dynamic ranges (e.g.,more than 80 dB), such circuits have a detection bandwidth, or speed,that may change depending on optical power level. At high optical powerlevels, a gain of the logarithmic amplifier circuit may be set low, anda resultant photodetection circuit may have a high detection bandwidth(e.g., higher than 300 kHz). Thus, high-frequency changes in thedetected photocurrent may be faithfully amplified, with low distortion.However, when operating with very low light levels of detectedphotocurrent, the bandwidth of the logarithmic amplifier circuit may beseverely degraded, and high-speed changes of the photocurrent may not beamplified faithfully at high-speed. The amplified output of thelogarithmic amplifier circuit may suffer from distortion and may be slowto respond.

In some examples of the present disclosure, the challenges ofconventional photodetection amplifiers are mitigated by providing adual-path amplification with selection between the paths based on aninput optical power threshold. One amplification path may include alinear amplifier circuit, for example, an operational amplifier circuit,coupled in series to a logarithmic amplifier circuit. A parallelamplification path may include a separate logarithmic amplifier circuit.A selection circuit may select outputs of either amplification pathdepending on a power threshold resulting in faithful amplification ofthe input signal across the frequency band even for low optical powerlevels. In some examples, the selection may occur post analog-digitalconversion of sampled outputs of the amplification paths.

FIG. 1 illustrates a block diagram 100 of a detection portion of anoptical measurement device, according to an example. In some examples,the block diagram 100 may depict a photodiode 102 coupled to anamplification block 110, which may include a first amplification pathincluding logarithmic amplifier circuit 114 and a second amplificationpath including an operational amplifier circuit 112 (linearamplification) and logarithmic amplifier circuit 116. Outputs of theamplification paths may be provided to a sampling circuit 104, where theoutputs may be converted from analog to digital. The digital outputs maybe provided to a selection circuit 106, which may select one of theamplification path outputs depending on a power threshold. An output ofthe selection circuit may then be provided to operational circuits anddevices of an optical measurement system such as an optical power meter,an optical spectrum analyzer (OSA), a fiberoptic monitoring system, orsimilar ones.

It should be appreciated that photodiode 102 may be formed as a p-njunction or a p-i-n (PIN) structure, where current is produced inresponse to illumination of a diode surface. When a photon of sufficientenergy is absorbed by the diode, it creates an electron—hole pair (innerphotoelectric effect). If the absorption occurs in the junction'sdepletion region, the carriers may be swept from the junction by thebuilt-in electric field of the depletion region. Thus, holes may movetoward the anode, and electrons toward the cathode, thereby creating aphotocurrent signal. Photodiode 102 may also include an optical filter,an optical lens, a fiberoptic coupling window, or similar additionalfeatures. As photocurrents tend to be low-level currents, one or morestages of amplification may be implemented depending on an instrumenttype.

Accordingly, the photocurrent signal from the photodiode 102 may beamplified by the amplification block 110, in some examples. A firstamplification path of the amplification block 110 may include the firstlogarithmic amplifier circuit 114. The second amplification path mayinclude the operational amplifier circuit 112 and the second logarithmicamplifier circuit 116 coupled in series. The operational amplifiercircuit 112 is an implementation of a linear amplifier. As such, theoperational amplifier circuit 112 may provide switchable gain allowinglow levels of input signal (photocurrent signal) to be amplifiedfaithfully. Faithful amplification refers to a circuit performance,where an amplified output signal follows a shape of an input signal.However, a dynamic range of the operational amplifier circuit 112 may belimited (e.g., about 30 dB) resulting in degradation (i.e., noamplification) at higher levels of the input signal. The firstlogarithmic amplifier circuit 114 and the second logarithmic amplifiercircuit 116 may have a larger dynamic range (e.g., about 80 dB or more),but a standalone logarithmic amplifier circuit may not be able toamplify the input signal faithfully at low levels resulting indegradation in such regions.

In some examples, the logarithmic amplifier circuit 114 forming thefirst amplification path may provide faithful amplification for higherlevel input signals, that is when a power of the input signal is above aparticular threshold. The combination of the operational amplifiercircuit 112 and the logarithmic amplifier circuit 116 forming the secondamplification path may provide relatively faithful amplification forlower-level input signals, while cutting off the signal at high powerlevels. The first and second amplification paths may amplify the inputsignal in parallel. The sampling circuit 104 may sample the outputs ofthe first and second amplification paths and perform analog-digitalconversion. Subsequently, the selection circuit 106, which may beimplemented as a field programmable gate array (FPGA) or a similarsignal processing circuit, may select one of the digitized outputsignals based on the power level.

Accordingly, the combination of the selected outputs may provide afaithfully amplified signal based on the photocurrent signal from thephotodiode 102 with a wide dynamic range that allows low levels of thephotocurrent signal to be amplified (and digitized) along with higherlevels. The selected output signal may be provided to operationalcircuits and/or devices in an optical measurement device such as anoptical spectrum analyzer (OSA), a swept wavelength measurement system,an optical power meter, a high-density optical power meter, a fiberopticmonitoring system, or similar ones.

In some examples, the sampling circuit 104 may include two sub-circuits,one for sampling and one for analog-digital conversion. Theanalog-digital conversion sub-circuit may convert respective sampledoutputs of the first and second amplification paths (first and secondlogarithmic amplifier circuits) to a first digital signal and a seconddigital signal. The selection circuit 106 may then select between thefirst and second digital signals. Yet, in other examples, theanalog-digital conversion may not be performed, and the selection may beperformed on analog output signals of the first and second amplificationpaths.

FIG. 2 illustrates a schematic diagram 200 of an amplification portionof an optical measurement device, according to an example. As shown indiagram 200, a first amplification path may include a logarithmicamplifier circuit 114, where an anode output A of a photodiode 102 iscoupled to a second input 12 of the logarithmic amplifier circuit 114 inparallel with a reference voltage VREF through a resistor R2. A firstinput 11 of the logarithmic amplifier circuit 114 may be coupled to thereference voltage VREF through a resistor R1. A logarithmic voltageoutput VLOG of the logarithmic amplifier circuit 114 may be coupledthrough resistors R4 and R5 (coupled in series) to an output of one ofthe internal amplifiers of the logarithmic amplifier circuit 114providing a scaled logarithmic output voltage VLOG_SCALED1. Resistors R4and R5 may be selected to adjust a gain of the logarithmic amplifiercircuit 114.

In some examples, a cathode output K of the photodiode 102 may becoupled to a negative input of the operational amplifier circuit 112. Apositive input of the operational amplifier circuit 112 may be coupledto a DC voltage source V6 to apply a bias voltage (if desired) to thephotodiode, or to trim out offset voltages in the operational amplifiercircuit 112. Resistors R8, coupled to an output of the operationalamplifier circuit 112, and R7 coupled between the output and thenegative input of the operational amplifier circuit 112, may providegain adjustment for the operational amplifier circuit 112 along withcapacitors C1, C3, and C4. Resistor R8 may provide the output of theoperational amplifier circuit 112 to a second input of the logarithmicamplifier circuit 116 in parallel with the reference voltage VREFthrough resistor R11. As with the logarithmic amplifier circuit 114, afirst input of the logarithmic amplifier circuit 116 may receive thereference voltage VREF through resistor R3. A logarithmic voltage outputVLOG of the logarithmic amplifier circuit 116 may be coupled throughresistors R6 and R10 (coupled in series) to an output of one of theinternal amplifiers of the logarithmic amplifier circuit 116 providing ascaled logarithmic output voltage VLOG_SCALED2. Resistors R6 and R10 maybe selected to adjust a gain of the logarithmic amplifier circuit 116.

As discussed in conjunction with FIG. 1 , an output of the logarithmicamplifier circuit 114 (VLOG_SCALED1) and an output of the logarithmicamplifier circuit 116 (VLOG_SCALED2) may be sampled and digitized by asampling circuit. A selector circuit may select between VLOG_SCALED1 andVLOG_SCALED2 based on a first power threshold and a second power levelthreshold (as the input signal is increasing to its peak and decreasingfrom its peak). The first and second power thresholds may have differentvalues or have the same value and may be selected based, at least inpart, on a saturation level of the operational amplifier circuit 112(linear amplifier).

In an implementation example, various components of the circuits shownin diagram 200 may have following values. R1=R3=499 kΩ, R2=R11=20 MΩ,R4=R6=100 kΩ, R5=R10=24 kΩ, R7=3 MΩ, R8=1.16 kΩ; C1=C3=15 pF, C4=2 nF.Such an implementation example may have a dynamic range in excess of 80dB and maintain a minimum bandwidth larger than 300 kHz over the entirepower range.

While specific circuit configurations such as the arrangements ofcapacitors and resistors are shown in conjunction with the operationalamplifier circuit 112 and logarithmic amplifier circuits 114, 116 indiagram 200, the illustrated configurations are not intended to belimiting. A dual-path amplification block may be implemented with otherconfigurations and component values using the principles describedherein.

FIG. 3 illustrates a block diagram of an optical system 300 including anoptical source or signal to be measured and a measurement system thatmay employ a linear amplifier and a logarithmic amplifier detector,according to an example. Optical system 300 may include an opticalsource or signal to be measured 302 with a light source 304. The lightfrom the optical source or signal to be measured 302 (or any otherlight) may be detected by a photodiode 308 of a measurement system 306.The measurement system 306 may be any optical measurement instrumentincluding, but not limited to, an optical spectrum analyzer (OSA), aswept wavelength measurement system, an optical power meter, ahigh-density optical power meter, a fiberoptic monitoring system, andsimilar ones. The measurement system 306 may include a pre-amplificationblock 310 to receive the detected signal (photocurrent signal) from thephotodiode 308. The pre-amplification block 310 may include, in someexamples, an operational amplifier circuit 312.

In some examples, the pre-amplification block 310 for adual-amplification path configuration may provide the detected signaldirectly and through the operational amplifier circuit 312 to anamplification block 314. The amplification block 314 may include twoparallel logarithmic amplifier circuits 316 and 318. One of thelogarithmic amplifier circuits (316) may receive the detected signaldirectly, while the other logarithmic amplifier circuit (318) mayreceive an output of the operational amplifier circuit 312. The dualoutputs of the amplification block 314 may be provided topost-amplification circuits 320, which may include a sampling circuit,an analog-digital (ND) conversion circuit, and/or a selection circuit322. The parallel amplification outputs may be sampled, converted todigital signals, and one of them selected based on a power levelthreshold of the input signal.

It should be appreciated that by selecting one of the amplificationpaths depending on the input signal power level, a faithfulamplification of the input signal over a wide dynamic range (e.g., morethan 80 dB) and with sufficient bandwidth to cover the input signal maybe achieved. Thus, low power level optical signals with high frequenciesmay be detected and amplified with similar or same fidelity as higherpower optical signals without any degradation in the amplified signal.An output of the post-amplification circuits 320 may be provided tooperational circuits and devices 330 of the measurement system 306,which may include, but are not limited to, display devices, measurementcircuits, storage devices, and comparable ones.

Some advantages and benefits of the systems and methods are readilyapparent. For example, the systems and methods using one or more of theconfigurations described herein may allow for a wide dynamic range,large bandwidth amplification of optical signals. By selecting parallel,dual amplification outputs, faithful amplification across an entirerange of input signal power levels may be achieved. Furthermore,discontinuities due to gain switching may also be avoided. Yetadditional advantages may include reduced complexity and cost of opticalmeasurement systems by reduced number of components (e.g., oneoperational amplifier and two logarithmic amplifiers) in place of moresophisticated, complex detection and amplification systems.

FIGS. 4A-4B illustrate graphs 400A and 400B representing input andoutput signals of a linear amplifier, a logarithmic amplifier, and acombination of thereof, according to an example. Graph 400A in FIG. 4Ashows input signal 406 (photocurrent signal), linear amplifier(implemented as operational amplifier) output 410, and logarithmicamplifier output 408 across time axis 404 (seconds) and power level axis402 (dB). A first and second power threshold 418 and 419 divide themeasurement into three regions 412, 414, and 416.

As shown in graph 400A, the input signal 406 increases with timereaching a peak (approximately centered in region 414) and thendecreases with time. The linear amplifier output 410 (e.g., an output ofan operational amplifier) may follow the shape of the input signal 406relatively closely in regions 412 and 416. However, because of thedynamic range limitation of the operational amplifier circuits, theamplification in region 414 may reach saturation and the linearamplifier output 410 may not follow the input signal 406. Thelogarithmic amplifier output 408, on the other hand, may follow theinput signal 406 closely (faithful amplification) in region 414 due tohigh dynamic range of logarithmic amplifier circuits. However,logarithmic amplifier circuits may have a slower response compared tolinear amplifier circuits, thus a more limited bandwidth. Therefore, thelogarithmic amplifier output 408 may lag compared to the input signal406 in region 412. The degradation (lag) in the logarithmic amplifieroutput 408 may be even larger in region 416.

Accordingly, a dual-amplification circuit for optical measurementinstrumentation may include a first amplification path with the firstlogarithmic amplifier circuit 114 and a second amplification path withthe operational amplifier circuit 112 and the second logarithmicamplifier circuit 116 (coupled in series). In some examples, two powerthresholds may be set for switching between the two amplification paths.As the input signal 406 increases, a first power threshold 418 may beset about a point where the operational amplifier circuit 112 goes intosaturation. In region 412 prior to the first power threshold 418, thesecond amplification path including the operational amplifier circuit112 and the second logarithmic amplifier circuit 116 may be used. Aresponse of the second amplification path is shown in graph 400B anddiscussed below in conjunction with FIG. 4B. Past the first powerthreshold 418, in region 414, the first amplification path including thefirst logarithmic amplifier circuit 114 may be selected. Thus, theamplified output may follow a shape of the input signal 406 closely inboth regions 412 and 414.

In some examples, as the input signal 406 decreases (beyond the peak)and drops below the saturation point of the operational amplifiercircuit 112, a second power threshold 419 may be set and the secondamplification path selected again past the second power threshold 419 inregion 416. While the second amplification path may not amplify theinput signal 406 in region 416 as faithfully as in region 412, the shapeof the output signal is still better than the first amplification path.Thus, an overall faithful amplification with the much higher dynamicrange of the logarithmic amplifiers may be achieved through theselection of the different amplification paths based on the first powerthreshold 418 and the second power threshold 419.

In an implementation example configuration, whose performance is shownin graphs 400A, the input signal may start at about −1.6 dB and peak atabout 1.6 dB. The operational amplifier circuit 112 may reach saturationat about 0.9 dB and drop out of saturation at about the same powerlevel. Thus, the first power threshold 418 and the second powerthreshold 419 may be set at 0.9 dB. While the first power threshold 418and the second power threshold 419 may be set at different power levelsdepending on the linear amplifier circuit characteristics, thethresholds may also be set to the same value in some implementations.

Graph 400B in FIG. 4B shows input signal 426, which is similar to theinput signal 406 in Graph 400A, compared to a combined output signal428, which results from selection of the dual-amplification outputs asdiscussed herein, across time axis 404 (seconds) and power level axis402 (dB). As shown in the graph 400B, the selection of the secondamplification path in region 412 as the input signal 426 increases up tothe first power threshold 418 and again in region 416 as the inputsignal 426 decreases from the second power threshold 419, and the firstamplification path in region 414 between the two power thresholdsprovides the combined output signal 428. Through fast switching betweenthe first and second amplification paths, the combined output signal 428is continuous and follows the shape of the input signal 426 through theentire dynamic range. While the combined output signal 428 has a smalllag at lower power levels as the input signal decreases (region 416), itgenerally follows a shape of the input signal 426 closely at high andlow power levels providing sufficient dynamic range and bandwidth forphotocurrent signal from a photodiode.

FIG. 5 illustrates a conceptual diagram illustrating selection ofdifferent amplifier configurations based on a power threshold, accordingto an example. The conceptual diagram in FIG. 5 includes illustrationsof three amplification scenarios. In a first scenario 500A, the inputsignal (photocurrent signal) is in region 412, where a power level ofthe input signal has not reached a power threshold for the operationalamplifier circuit 112 to enter saturation. Accordingly, the selectioncircuit 106 may select the first amplification path including theoperational amplifier circuit 112 and logarithmic amplifier circuit 116,where the output signal closely follows the input signal.

In some examples, as shown in second scenario 500B, the input signal(photocurrent signal) is in region 414, where the operational amplifiercircuit 112 may enter saturation and its output may substantially differfrom the input signal. Accordingly, the selection circuit 106 may selectthe second amplification path including the logarithmic amplifiercircuit 114, whose output may very closely follow the input signalproviding faithful amplification at high power levels.

In some examples, as shown in third scenario 500C, the input signal(photocurrent signal) is in region 416, where the input signal may dropbelow the power level threshold, where the operational amplifier circuit112 is in saturation. Accordingly, the selection circuit 106 may selectthe first amplification path including operational amplifier circuit 112and the logarithmic amplifier circuit 114, whose combined output mayfollow the input signal relatively closely.

Accordingly, the selection of different amplification paths fordifferent power levels of the input signal may provide faithfulamplification of the photocurrent input signal at low- and high-powerlevels allowing a high dynamic range amplifier with sufficient bandwidthfor optical input signals.

FIG. 6 illustrates a flow chart of a method for employing a combinationof linear and logarithmic amplifiers to amplify optical signals,according to an example. The method 600 is provided by way of example,as there may be a variety of ways to carry out the method describedherein. Although the method 600 is primarily described as beingperformed by the circuits of FIGS. 1 and 2 , the method 600 may beexecuted or otherwise performed by one or more processing components ofanother system or a combination of systems. Each block shown in FIG. 6may further represent one or more processes, methods, or subroutines,and one or more of the blocks (e.g., the selection process) may includemachine readable instructions stored on a non-transitory computerreadable medium and executed by a processor or other type of processingcircuit to perform one or more operations described herein.

At block 602 an anode output of the photodiode 102 may be received atthe first logarithmic amplifier circuit 114 and amplified by the firstlogarithmic amplifier circuit 114. The first logarithmic amplifier 114may be referred to as the first amplification path. As discussed herein,the output of the logarithmic amplifier circuit 114 may not follow theinput signal closely at lower power levels of the input signal butperform faithful amplification for higher power levels.

In a parallel process, at block 604, a cathode output of the photodiode102 may be received at the operational amplifier circuit 112 andamplified by the operational amplifier circuit 112. An output of theoperational amplifier circuit 112 may be received by the secondlogarithmic amplifier circuit 116 at block 606 and further amplified.The operational amplifier circuit 112 and the second logarithmicamplifier circuit 116 may be referred to as the second amplificationpath.

The outputs of the first and second amplification paths from parallelprocesses or block 602 and blocks 604, 606 may be sampled by thesampling circuit 104 at block 608. At optional block 610, the sampledoutputs of both amplification paths may be digitized and provided to theselection circuit 106. In some examples, the outputs may be provided tothe selection circuit in analog form without the analog-digitalconversion. The selection circuit 106 may be a field programmable gatearray (FPGA) or similar digital processing circuit.

At block 612, the selection circuit 106 may select one of the outputsfor forwarding to operational circuits or devices of an opticalmeasurement system based on a power level of the input signal. Asdiscussed herein, the first amplification path including the firstlogarithmic amplifier circuit 114 may be used at higher power levels,and the second amplification path including the operational amplifiercircuit 112 and the second logarithmic amplifier circuit 116 may be usedat lower power levels.

According to some examples, an optical amplification system may includea first amplification path including a first logarithmic amplifiercircuit to receive an input signal; a second amplification pathincluding a linear amplifier circuit and a second logarithmic amplifiercircuit coupled in series with the linear amplifier circuit, where thelinear amplifier circuit may receive the input signal; and a selectioncircuit. The selection circuit may select an output of the secondamplification path while the input signal is increasing and below afirst power threshold; select an output of the first amplification pathwhile the input signal is between the first power threshold and a secondpower threshold; select the output of the second amplification pathagain while the input signal is decreasing and below the second powerthreshold; and provide the selected outputs to a measurement circuit ordevice.

According to some examples, the optical amplification system may furtherinclude a sampling circuit, which may receive an output of the firstlogarithmic amplifier circuit and an output of the second logarithmicamplifier circuit; and sample the output of the first logarithmicamplifier circuit and the output of the second logarithmic amplifiercircuit. The optical amplification system may also include ananalog-digital conversion circuit, which may receive the sampled outputof the first logarithmic amplifier circuit and the sampled output of thesecond logarithmic amplifier circuit; convert the sampled output of thefirst logarithmic amplifier circuit and the sampled output of the secondlogarithmic amplifier circuit to a first digital signal and a seconddigital signal; and provide the first digital signal and the seconddigital signal to the selection circuit.

According to some examples, the linear amplifier circuit may be anoperational amplifier circuit. The first logarithmic amplifier circuitmay receive the input signal from an anode output of a photodiode, andthe linear amplifier circuit may receive the input signal from a cathodeoutput of the photodiode. The measurement circuit or device may be anoptical spectrum analyzer (OSA), an optical power meter, or a fiberopticmonitoring system. The first amplification path and the secondamplification path may amplify the input signal in parallel. The firstpower threshold and the second power threshold may be selected based, atleast in part, on a saturation level of the linear amplifier circuit.

According to some examples, a method for providing a dual-pathamplification of optical input signals for optical measurements mayinclude receiving, from a photodiode, an input signal at an input of afirst logarithmic amplifier circuit in a first amplification path; andreceiving, from the photodiode, the input signal at an input of a linearamplifier circuit in a second amplification path, where the linearamplifier circuit is coupled in series with a second logarithmicamplifier circuit in the second amplification path. The method may alsoinclude selecting, using a selection circuit, an output of the secondamplification path while the input signal is increasing and below afirst power threshold, an output of the first amplification path whilethe input signal is between the first power threshold and a second powerthreshold, and the output of the second amplification path again whilethe input signal is decreasing and below the second power threshold; andproviding the selected outputs to a measurement circuit or device.

According to some examples, the method may further include receiving, ata sampling circuit, an output of the first logarithmic amplifier circuitand an output of the second logarithmic amplifier circuit; and sampling,using the sampling circuit, the output of the first logarithmicamplifier circuit and the output of the second logarithmic amplifiercircuit. The method may also include receiving, at an analog-digitalconversion circuit, the sampled output of the first logarithmicamplifier circuit and the sampled output of the second logarithmicamplifier circuit; converting, at the analog-digital conversion circuit,the sampled output of the first logarithmic amplifier circuit and thesampled output of the second logarithmic amplifier circuit to a firstdigital signal and a second digital signal; and providing the firstdigital signal and the second digital signal to the selection circuit.

According to some examples, receiving the input signal at the input ofthe first logarithmic amplifier may include receiving the input signalfrom an anode output of the photodiode; and receiving the input signalat the input of the linear amplifier circuit may include receiving theinput signal from a cathode output of the photodiode. The method mayfurther include amplifying the input signal in the first amplificationpath and in the second amplification path in parallel. The method mayalso include selecting the first power threshold and the second powerthreshold based, at least in part, on a saturation level of the linearamplifier circuit. The linear amplifier circuit may be an operationalamplifier circuit.

According to some examples, an optical measurement system may include aphotodiode to detect an optical signal and, responsive to the detectedoptical signal, provide a photocurrent signal; and an amplificationblock to amplify the photocurrent signal. The amplification block mayinclude a first amplification path including a first logarithmicamplifier circuit to receive the photocurrent signal; and a secondamplification path including a linear amplifier circuit and a secondlogarithmic amplifier circuit coupled in series with the linearamplifier circuit, where the linear amplifier circuit may receive thephotocurrent signal. The optical measurement system may also include asampling circuit, which may receive an output of the first logarithmicamplifier circuit and an output of the second logarithmic amplifiercircuit; and sample the output of the first logarithmic amplifiercircuit and the output of the second logarithmic amplifier circuit. Theoptical measurement system may further include a selection circuit,which may select one of the sampled output of the first logarithmicamplifier circuit and the sampled output of the second logarithmicamplifier circuit based, at least in part, on a power level of thephotocurrent signal; and provide the selected output to a measurementcircuit or device of the optical measurement system.

According to some examples, the optical measurement system may furtherinclude an analog-digital conversion circuit, which may receive thesampled output of the first logarithmic amplifier circuit and thesampled output of the second logarithmic amplifier circuit; convert thesampled output of the first logarithmic amplifier circuit and thesampled output of the second logarithmic amplifier circuit to a firstdigital signal and a second digital signal; and provide the firstdigital signal and the second digital signal to the selection circuit.The first logarithmic amplifier circuit may receive the photocurrentsignal from an anode output of the photodiode; and the linear amplifiercircuit may receive the photocurrent signal from a cathode output of thephotodiode. The selection circuit may compare the power level of thephotocurrent signal to a saturation level of the linear amplifiercircuit.

It should be appreciated that while examples are described with twoamplification paths, one with a logarithmic amplifier circuit only, onewith an operational amplifier and a logarithmic amplifier, otherimplementations may also include different configurations. For example,multiple amplifier circuits may be used in one or both amplificationpaths. Each amplifier circuit may be configured (e.g., gain, bandwidth,etc.) according to specific device configuration or expected inputsignal levels. Furthermore, the amplification block described herein maybe employed to amplify signals other than optical signals (photocurrentsignal) as well.

While examples described herein are directed to configurations as shown,it should be appreciated that any of the components described ormentioned herein may be altered, changed, replaced, or modified, insize, shape, and numbers, or material, depending on application or usecase, and adjusted for desired resolution or optimal measurementresults.

It should be appreciated that the apparatuses, systems, and methodsdescribed herein may minimize, reduce, and/or eliminate amplificationdegradation in form of dynamic range and/or bandwidth, and therebyfacilitate more reliable and accurate optical measurements, specificallyfor low power level input signals. It should also be appreciated thatthe apparatuses, systems, and methods, as described herein, may alsoinclude, or communicate with other components not shown. For example,these may include external processors, counters, analyzers, computingdevices, and other measuring devices or systems. This may also includemiddleware (not shown) as well. The middleware may include softwarehosted by one or more servers or devices. Furthermore, it should beappreciated that some of the middleware or servers may or may not beneeded to achieve functionality. Other types of servers, middleware,systems, platforms, and applications not shown may also be provided atthe backend to facilitate the features and functionalities of thetesting and measurement system.

Moreover, single components may be provided as multiple components, andvice versa, to perform the functions and features described herein. Itshould be appreciated that the components of the system described hereinmay operate in partial or full capacity, or it may be removed entirely.It should also be appreciated that analytics and processing techniquesdescribed herein with respect to the optical measurements, for example,may also be performed partially or in full by other various componentsof the overall system.

It should be appreciated that data stores may also be provided to theapparatuses, systems, and methods described herein, and may includevolatile and/or nonvolatile data storage that may store data andsoftware or firmware including machine-readable instructions. Thesoftware or firmware may include subroutines or applications thatperform the functions of the measurement system and/or run one or moreapplication that utilize data from the measurement or othercommunicatively coupled system.

The various components, circuits, elements, components, and interfaces,may be any number of mechanical, electrical, hardware, network, orsoftware components, circuits, elements, and interfaces that serves tofacilitate communication, exchange, and analysis data between any numberof or combination of equipment, protocol layers, or applications. Forexample, the components described herein may each include a network orcommunication interface to communicate with other servers, devices,components or network elements via a network or other communicationprotocol.

Although examples are directed to test and measurement systems, such asoptical spectrum analyzers (OSAs), it should be appreciated that thesystems and methods described herein may also be used in other varioussystems and other implementations. For example, these may includevarious test, monitoring, and measurement systems. In fact, there may benumerous applications in optical communication networks and fiber sensorsystems that could employ the systems and methods as well.

What has been described and illustrated herein are examples of thedisclosure along with some variations. The terms, descriptions, andfigures used herein are set forth by way of illustration only and arenot meant as limitations. Many variations are possible within the scopeof the disclosure, which is intended to be defined by the followingclaims—and their equivalents—in which all terms are meant in theirbroadest reasonable sense unless otherwise indicated.

1. An optical amplification system, comprising: a first amplificationpath comprising a first logarithmic amplifier circuit to receive aninput signal; a second amplification path comprising a linear amplifiercircuit and a second logarithmic amplifier circuit coupled in serieswith the linear amplifier circuit, wherein the linear amplifier circuitis to receive the input signal; and a selection circuit to: select anoutput of the second amplification path while the input signal isincreasing and below a first power threshold; select an output of thefirst amplification path while the input signal is between the firstpower threshold and a second power threshold; select the output of thesecond amplification path again while the input signal is decreasing andbelow the second power threshold; and provide the selected outputs to ameasurement circuit or device.
 2. The optical amplification system ofclaim 1, further comprising: a sampling circuit to: receive an output ofthe first logarithmic amplifier circuit and an output of the secondlogarithmic amplifier circuit; and sample the output of the firstlogarithmic amplifier circuit and the output of the second logarithmicamplifier circuit.
 3. The optical amplification system of claim 2,further comprising: an analog-digital conversion circuit to: receive thesampled output of the first logarithmic amplifier circuit and thesampled output of the second logarithmic amplifier circuit; convert thesampled output of the first logarithmic amplifier circuit and thesampled output of the second logarithmic amplifier circuit to a firstdigital signal and a second digital signal; and provide the firstdigital signal and the second digital signal to the selection circuit.4. The optical amplification system of claim 1, wherein the linearamplifier circuit is an operational amplifier circuit.
 5. The opticalamplification system of claim 1, wherein the first logarithmic amplifiercircuit is to receive the input signal from an anode output of aphotodiode.
 6. The optical amplification system of claim 5, wherein thelinear amplifier circuit is to receive the input signal from a cathodeoutput of the photodiode.
 7. The optical amplification system of claim1, wherein the measurement circuit or device is one of an opticalspectrum analyzer (OSA), an optical power meter, or a fiberopticmonitoring system.
 8. The optical amplification system of claim 1,wherein the first amplification path and the second amplification pathare to amplify the input signal in parallel.
 9. The opticalamplification system of claim 1, wherein the first power threshold andthe second power threshold are selected based, at least in part, on asaturation level of the linear amplifier circuit.
 10. A method forproviding a dual-path amplification of optical input signals for opticalmeasurements, the method comprising: receiving, from a photodiode, aninput signal at an input of a first logarithmic amplifier circuit in afirst amplification path; receiving, from the photodiode, the inputsignal at an input of a linear amplifier circuit in a secondamplification path, wherein the linear amplifier circuit is coupled inseries with a second logarithmic amplifier circuit in the secondamplification path; selecting, using a selection circuit, an output ofthe second amplification path while the input signal is increasing andbelow a first power threshold, an output of the first amplification pathwhile the input signal is between the first power threshold and a secondpower threshold, and the output of the second amplification path againwhile the input signal is decreasing and below the second powerthreshold; and providing the selected outputs to a measurement circuitor device.
 11. The method of claim 10, further comprising: receiving, ata sampling circuit, an output of the first logarithmic amplifier circuitand an output of the second logarithmic amplifier circuit; and sampling,using the sampling circuit, the output of the first logarithmicamplifier circuit and the output of the second logarithmic amplifiercircuit.
 12. The method of claim 11, further comprising: receiving, atan analog-digital conversion circuit, the sampled output of the firstlogarithmic amplifier circuit and the sampled output of the secondlogarithmic amplifier circuit; converting, at the analog-digitalconversion circuit, the sampled output of the first logarithmicamplifier circuit and the sampled output of the second logarithmicamplifier circuit to a first digital signal and a second digital signal;and providing the first digital signal and the second digital signal tothe selection circuit.
 13. The method of claim 10, wherein receiving theinput signal at the input of the first logarithmic amplifier circuitcomprises receiving the input signal from an anode output of thephotodiode; and receiving the input signal at the input of the linearamplifier circuit comprises receiving the input signal from a cathodeoutput of the photodiode.
 14. The method of claim 10, furthercomprising: amplifying the input signal in the first amplification pathand in the second amplification path in parallel.
 15. The method ofclaim 10, further comprising: selecting the first power threshold andthe second power threshold based, at least in part, on a saturationlevel of the linear amplifier circuit.
 16. The method of claim 10,wherein the linear amplifier circuit is an operational amplifiercircuit.
 17. An optical measurement system, comprising: a photodiode todetect an optical signal and, responsive to the detected optical signal,provide a photocurrent signal; an amplification block to amplify thephotocurrent signal, the amplification block comprising: a firstamplification path comprising a first logarithmic amplifier circuit toreceive the photocurrent signal; and a second amplification pathcomprising a linear amplifier circuit and a second logarithmic amplifiercircuit coupled in series with the linear amplifier circuit, wherein thelinear amplifier circuit is to receive the photocurrent signal; asampling circuit to: receive an output of the first logarithmicamplifier circuit and an output of the second logarithmic amplifiercircuit; and sample the output of the first logarithmic amplifiercircuit and the output of the second logarithmic amplifier circuit; anda selection circuit to: select one of the sampled output of the firstlogarithmic amplifier circuit and the sampled output of the secondlogarithmic amplifier circuit based, at least in part, on a power levelof the photocurrent signal; and provide the selected output to ameasurement circuit or device of the optical measurement system.
 18. Theoptical measurement system of claim 17, further comprising: ananalog-digital conversion circuit to: receive the sampled output of thefirst logarithmic amplifier circuit and the sampled output of the secondlogarithmic amplifier circuit; convert the sampled output of the firstlogarithmic amplifier circuit and the sampled output of the secondlogarithmic amplifier circuit to a first digital signal and a seconddigital signal; and provide the first digital signal and the seconddigital signal to the selection circuit.
 19. The optical measurementsystem of claim 17, wherein the first logarithmic amplifier circuit isto receive the photocurrent signal from an anode output of thephotodiode; and the linear amplifier circuit is to receive thephotocurrent signal from a cathode output of the photodiode.
 20. Theoptical measurement system of claim 17, wherein the selection circuit isto compare the power level of the photocurrent signal to a saturationlevel of the linear amplifier circuit.